Ballast water refers to sea water that is charged into a ballast tank of a vessel to maintain balance of the vessel when the vessel sails without cargo. As marine transportation rates have gradually increased along with an increase in international trade, the number of vessels used has increased, and the vessels used are becoming larger. As a result, the amount of ballast water used in vessels has greatly increased. As the amount of ballast water used in vessels is increased, occurrence of damage to indigenous ocean ecosystems attributable to foreign marine creature species is also increased. To solve such international environmental issues, in 2004, the IMO (International Maritime Organization) established ‘International convention for the control and management for vessel's ballast water and sediments’. Since 2009, ballast water treatment apparatuses have been obligatorily installed in newly constructed vessels.
As examples of conventional methods for treating ballast water, a method of exchanging ballast water when a vessel is at the sea and a method of treating ballast water when a vessel is dry docked have been used. However, these conventional methods have the disadvantage of being inefficient. Therefore, recently, a method using a ballast water treatment apparatus installed in a vessel has been widely used. Particularly, a filtration method using a filter is mainly used as the ballast water treatment apparatus. As vessels are becoming larger, there is an increasing need for the treatment of a large amount of ballast water. Accordingly, multi-cage ballast water filtering apparatuses that can treat a large amount of ballast water are recently being used.
FIG. 1 is a partially broken exploded perspective view showing a conventional multi-cage type ballast water filtering apparatus. FIG. 2 is a plan view of the conventional multi-cage type ballast water filtering apparatus. FIG. 3 is a bottom view of the conventional multi-cage type ballast water filtering apparatus.
Hereinafter, the conventional multi-cage type ballast water filtering apparatus will be explained with reference to FIGS. 1 through 3.
The conventional multi-cage type ballast water filtering apparatus includes a first pressure sensor 73 that measures the pressure in space between filters 71 and the body 72, and second pressure sensors 75 that are installed in respective filtering units (referring to units each of which includes the single filter 71 and an automatic washing unit 74) and measure the pressures in the filters 71 of the respective filtering units. Based on the theory that foreign substances adhering to the filters 71 increase the pressure in the filters 71, if there are filtering units that cause differences between the pressure in the body 72 and the pressures in the filters 71, and that pressure exceeds a predetermined range, the automatic washing units 74 of the corresponding filtering units are successively operated to backwash the related filters 71. However, if the pressure in the filter 71 of any specific filtering unit that has been backwashed is rapidly increased again, only the filter 71 of the specific filtering unit may be repeatedly backwashed without the other filters 71 being backwashed.
Furthermore, in the conventional multi-cage type ballast water filtering apparatus, discharge pipes 76 through which foreign substances and backwash water are discharged out of the body 72 are welded to the inner surface of the body 72. Therefore, to apply corrosion-proof paint to the inner surface of the body 72 and the discharge pipes 76, a worker must enter the body 72 and conduct the corrosion-proof paint applying work after the discharge pipes 76 have been installed in the body 72. Therefore, it becomes difficult to conduct the painting work for preventing corrosion of the inner surface of the body, thus making installation and maintenance of the apparatus difficult.
Moreover, in the conventional multi-cage type ballast water filtering apparatus, the discharge pipes 76 protrude out of the body 72 and communicate with a single connection pipe 77. Thus, the external elements of the filtering apparatus are complex, and the size of the filtering apparatus is increased, making the installation and maintenance of the apparatus more difficult. Particularly, under special environment conditions, that is, in a vessel, the space provided to install such a filtering apparatus is very small (generally, to ensure sufficient space for the original purposes of a vessel, vessels are designed such that spaces such as a machinery room are relatively small). Therefore, given the fact that space defined outside the filtering apparatus is also small, the conventional structure in which installation positions of many elements are focused outside the filtering apparatus makes the installation and maintenance of the filtering apparatus more difficult. In addition, each discharge pipe 76 having a small cross-sectional area is comparatively long and bent. Hence, foreign substances may be easily caught in the discharge pipe 76, whereby backpressure impeding discharge of foreign substances and backwash water is easily formed.